The object of the present invention is an instrument for measuring linear magnitudes which comprises at least one feeler rigidly connected with a linear slideblock, at least one second feeler rigidly connected with a slider mounted for sliding on the slideblock, and at least one play take-up device connected to the slider and inserted between an inner resting face of the latter and a corresponding outer resting face of the slideblock within a space of rectangular cross section extending along the slider.
The play take-up device of the known instruments of this kind, such as, for instance, slide calipers and measurement columns are intended to compensate for the wear by friction caused by the displacements of the slider on the slideblock upon the taking of measurements.
This take-up of play is indispensable in order to assure precision of the measurement, since it avoids, upon the taking of a measurement by pressure of the feelers against the object to be measured, the tilting of the slider on the slideblock and thus distortion of the parallelism of the feelers, which would not fail to take place as a result of the offset of these feelers with respect to the slider and the slideblock.
Various types of play take-up devices are at present employed, depending on the degree of precision required.
Thus on certain medium-precision slide calipers, used, for instance, in locksmith shops, the play take-up device consists of a spring blade which is curved at its center and curved back and held at its two ends by the slider. The curved portion of this blade rests against an inner face of the slider and its two ends rest against a corresponding outer face of the slideblock. A set screw is engaged in a threaded hole in the slider and debouches on the inside onto the curved portion of the spring blade. This set screw makes it possible to change the pressure of the spring blade against the slideblock as desired and, at the end of the stroke, to lock the slider against the slideblock when it is desired, for instance, to retain the measurement after having freed the instrument from the part measured.
With this arrangement, the force of the spring blade is made sufficient to compensate for normal measurement pressure. However, there is still a doubt as to the precision of the measurement, since this pressure may accidentally be exceeded as, for instance, when making a difficult measurement in a narrow space, particularly when the spring blade has lost a part of its elasticity as a result of aging. In this case in fact nothing any longer prevents the slider from canting on the slideblock since the thickness of the spring blade is necessarily less than the height of the space which it occupies.
In another known type of play take-up device with leaf spring, applied to a measurement column having slightly offset feelers, the pressure of this blade is no longer applied directly at its ends to the slideblock but rather via a slide shoe of plastic material. The pressure of the spring blade is, in this case, distributed by the shoe in a more uniform manner, which constitutes an improvement. However, the aforesaid problems inherent in the use of a spring blade as play take-up means are still entirely present, despite the small offset of the measurement feelers.
On certain precision slide calipers used for the manufacture and inspection of parts in mechanical construction shops, the play take-up device consists of a thickness wedge of parallelpiped shape which is fitted for sliding in the space reserved for it between the slider and the slideblock. Two first set screws with heads countersunk in the slider are arranged resting against the two ends of the thickness wedge and are used for the periodic adjustment of the play take-up pressure. One of these two screws furthermore has a drive head engaged in a hole in the thickness wedge. The locking of the slider on the slideblock is assured, when desired, by a third set screw whose operating head rests against the central portion of the thickness wedge between the two screws with countersunk heads.
This device does not have the drawbacks of the preceding devices since, once the two screws with countersunk heads have been adjusted in such a manner as to eliminate the play between the thickness wedge and the slideblock, the slider can no longer cant under the effect of a strong measurement pressure. However, the use in shops of slide calipers equipped with devices of this type has shown a drawback inherent in the use of screws as take-up means. They can in fact loosen, without the operator noting this, as a result of vibrations caused, for instance, by the frame or an operating machine part on which the caliper has been temporarily placed. Furthermore, this device requires periodic adjustments of the two countersunk-head set screws as wear by friction between the thickness wedge and the slideblock takes place, in the absence of which the precision of the measurement would change with the passage of time.